His daughter Lucy, 38, and his first wife Jane, 65, had been at his bedside last night.

Professor Hawking was diagnosed at 21 with the muscle-wasting
disease, which has gradually robbed him of his voice and movement in
his limbs.

He speaks through a distinctive voice synthesiser and uses a motorised wheelchair.

The scientist was told he could expect to live for two years but
has defied this prediction to become Britain's oldest-known survivor of
motor neurone disease.

Professor Hawking returned from a work trip to America on Saturday after ill-health forced him to cancel appearances.

He was taken to Addenbrooke's hospital, Cambridge, yesterday lunchtime for tests which a close colleague said were 'far more serious than routine'.

A spokesman for Cambridge University, where he is Lucasian Professor
of Mathematics, said Professor Hawking had been unwell for a couple of
weeks.

Professor Hawking, who also has two sons Tim, 30, and Robert, 42,
and a grandchild, is due to retire from his post in October but planned
to continue to work at the university.

One of his last public appearances was last September, when he unveiled a £1million clock at Corpus Christi College, Cambridge.

Professor Peter Haynes said: 'He is a remarkable colleague. We all hope he will be among us again soon.'

Professor Hawking's complicated family life has also attracted
attention. In 1990, on his silver wedding anniversary, he left his wife
Jane to move in with Elaine Mason, a nurse who had cared for him. They
were married in 1995.

The scientist, pictured floating on a zero-gravity jet in 2007, is one of
the world's longest-surviving sufferers of motor neurone disease

The bitter split astonished colleagues and friends, and his new wife was later accused by his family of bullying him.

In 2000, detectives launched an inquiry after Professor Hawking
made a number of visits to Addenbrooke's suffering cuts and bruises.
Another inquiry was opened in 2003.

Professor Hawking denied that his second wife was to blame. The couple divorced three years ago.

In 1988 A Brief History of Time, which sold more than nine million copies and made him a household name, was published.

His fame led to guest roles in The Simpsons, Red Dwarf and Star Trek: The Next Generation.

One of his greatest achievements was the discovery of 'Hawking
radiation' - the way a black hole leaks energy and fades to nothing.

DIAGNOSIS CAME AS 'GREAT SHOCK' TO HAWKING

Professor Stephen Hawking suffers from amyotrophic lateral sclerosis (ALS), the most common form of motor neurone disease.

He has described receiving a diagnosis of ALS in his early 20s as a ‘great shock’.

People with ALS experience weakness and ‘wasting’ of the muscles in their limbs and early symptoms include tripping up when walking or dropping things.

The average life expectancy for somebody with ALS is two to five years from the time symptoms first appear.

The disease is progressive and people with ALS experience muscle weakness in their hands, arms or legs (or all three).

Professor Hawking with his former wife Jane in 1999 and right, with sons Robert and Tim in the 1980s

Twitching and ‘cramping’ of the muscles is also common, especially in the hands and feet.

In the more advanced stages, people often have difficulty speaking, swallowing or breathing and experience paralysis.

Not all people with ALS have the same symptoms or the same
progression of the disease but progressive muscle weakness and
paralysis are experienced by most.

Professor Hawking, who has had the disease for more than 40 years, first developed symptoms when he was 21.

and Tim in the 1980s

Brian Dickie, director of research at the Motor Neurone Disease
Association, said the fact Professor Hawking had lived for so long was
‘remarkable’.

He added: ‘ALS accounts for almost 90 per cent of cases of motor neurone disease.

‘Professor Hawking’s form of the disease is pretty progressive.

‘About 5 per cent of people with ALS will survive for 10 years or
longer - he really is at the extreme end of the scale when it comes to
survival.’

There is little treatment on offer for ALS. Only one drug - Rilutek
(also known as riluzole) - is licensed for the disease and typically
extends survival by just three to six months.

‘Apart from that, treatment is palliative, which means you are
treating the symptoms of the disease,’ Mr Dickie said. ‘We can’t stop
the disease progressing.’

Palliative treatments include ventilation systems to help with
breathing, feeding tubes if swallowing is a problem and muscle
relaxants for muscle cramping.

ALS is often referred to as a neurodegenerative disease because it affects nerve cells in the brain and the spinal cord.

Motor neurons from the brain to the spinal cord, and from the spinal
cord to the muscles, pass messages or ‘impulses’ around the body.

In ALS, the motor neurons degenerate and eventually die.

When the motor neurons die, the ability of the brain to initiate and control muscle movement is lost.

When muscles no longer receive messages from the motor neurons, they are said to ‘waste’.

The wasting is therefore a symptom of the disease rather than a cause.

Scientists do not know what causes more than 90 per cent of cases of
ALS. Around 5 per cent to 10 per cent of cases are hereditary and
research on genetics is progressing in this area.

An estimated 3,500 people in the UK have ALS.

On his website, Professor Hawking writes about living with the disease.

‘I am quite often asked: How do you feel about having ALS? The answer is, not a lot,’ he says.

‘I try to lead as normal a life as possible, and not think about my
condition, or regret the things it prevents me from doing, which are
not that many.’

Speaking about his diagnosis, he said: ‘It was a great shock to me to discover that I had motor neurone disease.

‘I had never been very well co-ordinated physically as a child. I
was not good at ball games, and my handwriting was the despair of my
teachers.

‘Maybe for this reason, I didn’t care much for sport or physical activities.

‘But things seemed to change when I went to Oxford at the age of 17.

‘I took up coxing and rowing. I was not Boat Race standard, but I got by at the level of inter-College competition.

‘In my third year at Oxford, however, I noticed that I seemed to be
getting more clumsy, and I fell over once or twice for no apparent
reason.

‘But it was not until I was at Cambridge, in the following year, that my father noticed and took me to the family doctor.

‘He referred me to a specialist and shortly after my 21st birthday I went into hospital for tests.

‘I was in for two weeks, during which I had a wide variety of tests.
They took a muscle sample from my arm, stuck electrodes into me, and
injected some radio opaque fluid into my spine, and watched it going up
and down with x-rays, as they tilted the bed.

‘After all that, they didn’t tell me what I had, except that it was not multiple sclerosis, and that I was an atypical case.

‘I gathered, however, that they expected it to continue to get
worse, and that there was nothing they could do, except give me
vitamins.’

He said the later realisation that he had an incurable disease, that
was likely to kill him within a few years ‘was a bit of a shock.

‘How could something like that happen to me? Why should I be cut off like this?

‘However, while I had been in hospital I had seen a boy I vaguely knew die of leukaemia in the bed opposite me.

‘It had not been a pretty sight. Clearly there were people who were worse off than me.

At least my condition didn’t make me feel sick. Whenever I feel inclined to be sorry for myself I remember that boy.’